Bi-orientation undergone by the container during its blow molding induces residual stresses in the material. Such residual stresses are released during hot-filling, causing a deformation of the container that might make it unsuitable for use—and hence for sale.
In order to decrease deformation of the container during hot-filling, it is known to heat set the container, whereby the blow molding is conducted within a mold provided with sidewalls heated at a predetermined temperature between 80° C. and 180° C., and the container is held against the sidewalls at the end of the blow molding for a predetermined time (generally several seconds).
However, heat setting solves only part of the problems of deformation of a hot-fill container. Indeed, while cooling, the product and the air above the liquid in the capped container undergo a decrease in volume that tends to make the container retract.
Several solutions have been considered for decreasing the visible effects of such retraction. These solutions generally concern the shape of the container.
For example, it has been proposed to equip the body of the container with deformable side panels that bend inwards under the effect of the retraction and bend back outwards when the container is opened. Such containers must be handled with care by the user because of the flexibility of the body, which may result in accidental spraying.
U.S. Pat. Appl. No. 2008/0047964 (Denner et al, assigned to CO2PAC) discloses a container comprising a pressure panel located in the bottom portion of the container.
According to Denner, the pressure panel is movable between an outwardly-inclined position and an inwardly-inclined position to compensate for a change of pressure inside the container. In order to alleviate all or a portion of the vacuum forces within the container, the pressure panel is moved from the outwardly-inclined position by a mechanical pusher after the container has been capped and cooled, in order to force the pressure panel into the inwardly-inclined position. The inversion of the pressure panel from the outwardly-inclined position to the inwardly-inclined position reduces the internal volume of the container.
However, Denner fails to address the problem of badly (either over—or insufficiently) filled containers, along with the problem of cap leakage. Such problems frequently arise with hot filling, wherein the air volume (also called headspace) between the product and the cap undergoes important variations of pressure. In particular, air pressure in the headspace during inversion of the pressure panel might exceed the sealing capability of the cap and thereby lead to undesired air leaks.